Plethysmography has been used to measure waveforms of the arterial vasculature. For example, U.S. Pat. No. 6,491,639 to Turcott, which is incorporated by reference herein, describes an implantable device with plethysmography circuitry for acquisition of such waveforms. Specifically, a waveform can provide arterial pulse amplitude. Depending on placement of the plethysmography sensor, a few tens to a few hundreds of milliseconds after a QRS complex, a waveform voltage may reach a minimum and start to increase as blood volume increases in the sensed arterioles. Where the sensor is positioned subcutaneously in a pectoral pocket directed away from the surface of the body, a wide-band voltage waveform reaches a peak after about 100 msec. In this example, the excursion from minimum to maximum represents the arterial pulse amplitude. During diastole the recoil of the elastic arterial vessels continues to force blood through the capillaries, so that blood flows through the capillary bed throughout the entire cardiac cycle. The decrease in blood volume on the arterial side during diastole results in the decrease in the plethysmography waveform during this period. For a narrow-band voltage waveform, the maximum may occur within about 100 msec of a QRS complex and the minimum thereafter within about 200 msec of the QRS complex. Again, in the foregoing example, a delay exists between cardiac action and the sensed voltage based in part on distance between the sensor and the heart.
As described herein, various exemplary techniques provide skin response information, optionally using a plethysmographic sensor. Such information may be applied to one or more types of therapy (e.g., cardiac therapies, neural therapies, etc.).